(1) Field of the Invention
The present invention relates to a system for efficient oil discharge from an engine.
(2) Prior Art
A typical engine bearing compartment is provided with oil through jets for the purpose of bearing lubrication and compartment cooling. A sealing airflow is provided in an upstream cavity and enters the bearing compartment through holes inside a rotating disc. Additional seal airflows are provided to the seals and prevent oil leakage out of the compartment's outer and inner rotor/stator interface.
In general, air and oil flows mix inside bearing compartments and generate a high velocity swirling flow pattern that forms a liquid wall film along the internal compartment walls. In the case of an oil film flow along a rotating wall, the oil film will be pumped by the centrifugal acceleration to the free end of the shaft where it will separate, disintegrate into droplets, and flow radially outwards until it coalesces on another surface. In the case of oil coalescence on a stationary surface, superimposed effects of interfacial shear and gravitational forces will dominate the oil film motion. In any bearing compartment cavity with rotating inner shaft and stationary outer wall, at some circumferential position downstream of bottom dead center (BDC), the oil film flow along the stationary wall will be exposed to counter-current effects of interfacial shear and gravitation. Gravitational forces tend to pull the oil film toward BDC, whereas interfacial shear tries to push the oil away from BDC. In addition, high interfacial shear will destabilize the liquid wall film flow and tends to entrain oil into the air stream. As a result, airflows that are supposed to be discharged through breather pipe(s) out of the bearing compartment always carry a certain amount of oil with them. In order to manage air and oil flows through bearing compartments efficiently, i.e. to maintain a positive seal pressure differential to prevent oil leakage and to minimize oil consumption and breather mist generation, low breather pipe oil content is desirable, especially at sub-idle and idle operation of the engine.
Thus, the bearing compartment has to be designed such that mixing air and oil is minimized. One element in achieving low breather pipe oil content is to reduce the residence time of the oil inside the bearing compartment by providing efficient means of scavenging the oil, and, therefore, minimizing the amount of oil that is exposed to the destabilizing effect of interfacial shear stresses.
A typical tangential scavenge port has scavenge scoops which are intended to discharge mainly oil and are usually located at or close to BDC. It is recognized however that due to strong air/oil interactions inside the bearing compartment cavities, oil film flows along the stationary surfaces usually contain significant air inclusion (bubbles) and a foamy air/oil layer close to the gas/liquid interface. The air content in the liquid film flow tends to increase flow area requirements for efficient discharge.
In order to connect the scavenge port with the plumbing of the lubrication system, the designer faces the challenge of providing means of directing a two-phase air/oil mixture with high circumferential flow velocity and significant velocity differences between both media into an axial or radial flow direction. In order to direct the swirling bearing compartment two phase air/oil mixture from a circumferential to an axial or radial exit pipe flow direction, current systems use tangential scoops that capture as much of the bearing cavity width as possible and transition into an integrated 90 degree bend that connects to the exit pipe. Due to minimum length requirements for the 90 degree bend, scavenge ports of this kind have an inlet plane that has to be located several degrees upstream of BDC. Oil that is provided to the bearing compartment cavity downstream of this inlet plane has to be carried by interfacial shear forces around the compartment and across Top-Dead-Center (TDC) until it can reach the inlet plane or it will collect in the bottom of the cavity. The former is usually achieved at high power settings, the latter is the dominant flow pattern at low power settings such as motoring, windmilling, or idle.
Since oil must be discharged efficiently at both low and high power regimes, the single scavenge port must be compromised slightly to work in both conditions. In some applications, two scavenge ports are used to capture oil at low power and high power. Because the fluid within the compartment is two phase air/oil, the two scavenge ports must be connected to separate pump stages to avoid loss of prime in the pump. If two scavenge ports are connected to a single pump stage, there is a propensity to scavenge only the lower density air, allowing the oil to puddle up within the compartment, create significant heat generation, and greatly increase the risk of oil leakage. It is therefore desirable to have a highly efficient scavenge port that works at low and high power with only a single pump stage, which is obviously lower in density and cost.
In order to allow drainage of oil that is not captured by the tangential scoop and collects in the sump of the compartment, drain holes are integrated into the tangential scoop/bend arrangement at BDC. This arrangement works satisfactorily for certain minimum compartment sump dimensions (radial distance between rotating shaft and outer stationary wall) and moderate rotational speeds. However, as size constraints for engine cores become more severe and engine speeds increase, limitations of this type of scavenge port arrangement become apparent—especially for cases where the compartment height approached the exit pipe diameter, which means that the tangential inlet scoop blocks the whole radial depth of the cavity. This blockage results in a severe reduction of interfacial shear, which would be required at high levels in order to drive all oil across TDC. The impact of these limitations depends strongly on the oil flow distribution at low power settings. As the size of the sump region decreases, the distance between the compartment seals and the free surface of the oil pool decreases, increasing the risk of oil leakage. This phenomenon is aggravated by the fact that the interfacial shear acting on the gas/liquid interface pushes oil away from the drain at BDC, forming a large recirculation zone several degrees downstream of BDC. This recirculation zone tends to contaminate the seals and causes oil leakage out of the compartment.